This invention concerns index color cathode ray tubes (CRTs) and, specifically, wide band index tube systems. The high resolution capability of the single beam index CRT is well known, and it has demonstrated great potential for widespread use in color monitors for computer terminals.
A conventional single beam index CRT includes a faceplate having a pattern of vertically oriented stripes of different colored light emitting phosphors on its inner surface, with each phosphor stripe being separated from its neighboring phosphor stripe by a guard band of inert material, that is, material that does not emit visible light under electron bombardment. The guard band enables selective energization of the different colored light emitting phosphors by a single electron beam and permits a larger electron beam for maximizing the brightness of the CRT. The phosphor stripes are arranged in a regular pattern interspersed with special signal areas, disposed at regular intervals on the screen, for generating index pulses in response to impingement by the scanned electron beam. The special signal areas may comprise narrow strips of energy emissive material overlying an aluminized layer deposited on the rear surface of the screen, that is, the surface closest to the electron gun. The energy emissive material, which may be a conventional monochrome phosphor such as type P47, produces ultraviolet light in response to electron bombardment that is directed toward the back of the CRT where it is sensed by a photo multiplier tube (PMT) positioned outside of and to the rear of the CRT envelope. The aluminized layer prevents visible light that may be emitted from the index strips from reaching the front of the tube. As the electron beam is deflected across the phosphor screen, the pulses of energy emitted by the index strips are detected by the PMT and processed by suitable circuitry to form an index signal.
Beam index CRT systems may be generally categorized as narrow band and wide band. In the more common narrow band system, the electron beam video excitation frequency is relatively fixed throughout each scanning line or beam traversal across the CRT screen, with only minor adjustments in phase being made. The narrow band system has the advantage of noise immunity and, consequently, can be operated with very low index strip excitation levels. Its disadvantage is that only relatively minor variations in time spacing between index strips can be compensated for. The result is often very poor color purity.
In a wide band index system, video excitation is controlled on a strip-by-strip basis, which automatically compensates for variations in timing of the index strips. Examples of wide band index systems are U.S. Pat. Nos. 4,408,223 and 4,468,690. Wide band systems afford very close control over the beam and can yield improved efficiency due to the precision with which the beam position is known. The major disadvantage of a wide band system is its susceptibility to noise. Thus, the excitation level of the index strips must generally be greater to produce an acceptable signal-to-noise ratio. The high index strip excitation adversely impacts the no video "blackness" of the tube and the contrast of the display. A technique for maintaining these characteristics is to turn the electron beam off between index strips, in the absence of video information. As discussed in copending application Ser. No. 738,797, a multi-level index signal is employed, with the electron beam being operated at a low level when positioned to impact an index strip, in the absence of video information, and at a higher level when positioned to impact a video strip in the presence of video information. This characteristic, in conjunction with turning the electron beam off between index strips under no-video conditions, enhances the contrast and purity of the display.
The above-mentioned patent addresses a difficulty that affects the brightness capability of an index tube. Under video conditions, the electron beam may impinge an index strip in addition to a color phosphor stripe. This occurs particularly under electron bombardment of the color stripes immediately adjacent to an index strip and imposes limits on the relative sizes of the index strip, guard bands, video stripes and electron beam. In the patent, the video signals corresponding to the color stripes on each side of the index strips are sensed. Correction means produce opposite polarity pulse signals that are added to the PMT output which corrects the developed index signal by offsetting the effects of adjacent video stripe excitation on the index strip. Under "heavy" video conditions, the index strip can still be found.
Rather than adding counteracting signals to the PMT output that is supplied to the index signal developing circuits, the present invention modifies the duration of the index video drive signal and incorporates closer location of increased amplitude video excitation drive signals adjacent each side of an index strip, respectively, to permit greater video excitation and achieve more brightness. Thus in the preferred embodiment of the invention, the video drive signals for the color phosphor stripes immediately adjacent the index strips are able to be tailored, i.e., increased in amplitude and moved closer to the index strip, relative to previous systems, because the index strip excitation level is high enough to permit the index signal to be accurately produced despite spillover from the adjacent color video drive signals. Further, the tailoring is such that a minimum amount of undesirable video index signal spillover onto adjacent unilluminated phosphor stripes occurs to avoid color contamination.